This invention relates in general to a weighing device, and more particularly to a weighing device which is especially suitable for automatically regulating a crystal-growing process or operation.
The growing of crystals is well known. For example, there is the so-called Czochralski-method in which a crystal melt is provided which is contacted by a suitable instrumentality, e.g. an already existing crystal, which is then slowly raised while being rotated. The existing crystal is sometimes called a "seeding" crystal. As the seeding crystal is rotated and slowly raised, a quantity of the melt is being drawn up with it and now forms a new crystal whose diameter is a very sensitive function of the temperature of the melt. Increasing melt temperature causes a decrease in the crystal diameter, and decrease in temperature causes an increase in the crystal diameter, i.e. the diameter of the crystal that is being produced.
In many instances, for example for quality reasons or for economic reasons, a precise diameter of sequence of diameters is prescribed over the longitudinal axis of the crystal, and no deviations from the diameter or from the sequence of diameters is permissible or desired. In such a case, the controlling of the crystal growth in order to assure the desired diameter or diameter sequence causes great difficulties for the operators of the crystal-growing equipment, because a control by visual observation of the boundary surface of the growing crystal and the melt, and an appropriate readjustment of the melt temperature by regulating the heat supplied to the melt, requires extreme care and much experience.
These conditions are aggravated by the fact that crystal growth in accordance with the Czochralski-method as a rule requires anywhere between several hours to several days; during this operation, which is required for the growth of a single crystal, the growth must be continuously supervised by experienced operators. In many instances, however, technical reasons make a visual observation of the crystal diameters impossible, or possible only with great difficulty, at least during certain stages of the growth process. These technical reasons have to do, for example, with a drop in the level of the melt or with the growth of crystals in accordance with the liquid encapsulated Czochralski-method under high pressure. Because of this, it is highly desirable to provide for an automatic regulation of the growing process in order to obtain the desired diameter control of the growing crystal, with a view not only of assuring the desired diameter or sequence of diameters along the crystal axis but also of reducing the operating cost in terms of the personnel required for supervision, and in addition of making it possible to operate the very expensive crystal growing apparatus on a more uniform time basis.
Arrangements for providing automatic regulation are already known. One of these arrangements regulates the growing process on the basis of the weight change of the growing crystal or of the melt from which the crystal is being withdrawn. The crystal or the melt are being weighed and a feed-back loop is provided in which the actual weight changes are compared with a reference value or values corresponding to the weight changes that should exist if the desired diameter or sequence of diameters are being obtained. Existing differences are then utilized for automatically controlling the supply of thermal energy to the melt.
If only simple crystal growing conditions are involved, the crucible containing the crystal melt can be placed directly upon a commercially available scale, together with the necessary baffles which provide for radial and axial shielding against thermal energy. However, in many instances the crucible must be constantly rotated during the crystal growing process in order to compensate for temperature gradients. If that is the case, then the necessary drive which causes the turning of the crucible influences the weighing results to such an extent that they are no longer reliable. In this connection it must be kept in mind that the weight variations between the actual weight and the reference weight are on the order of between several milligram and several gram, i.e. that they are very small and make high requirements of the resolution capability of the scale, so that they can be very readily influenced by the drive components needed to rotate the melt-containing crucible.
Moreover, if the crystal-growing space must be closed because the operation is carried out at vacuum or at high pressures, then scales of the type mentioned above cannot be used at all, because a weight-transmitting member of the scale would have to pass out of the sealed environment through an appropriate seal, and the friction between the movable component of the scale and the seal would be sufficient to cause a totally incorrect indication of the weight. On the other hand, it is not possible to reverse the relationship and to locate the scale in the sealed crystal-growing space itself, because the sclaes are too large for this purpose and the high pressures and aggressive vapors existing in the crystal-growing space would have deliterious influences upon the scale and on the operation of the scale.
As these problems were recognized, attempts were made to replace the scales by electronic force measuring cells. In principle, this can be done but difficulties again arise if the melt-containing crucible must be rotated. To some extent, these difficulties can be overcome by rotating the measuring cells together with the crucible. In that case, however, it is necessary to provide slip-ring contacts in order to supply electrical energy to and remove signals from the measuring cells. Such slip-ring contacts are subject to malfunction because of the generally corrosive environment in which they must be operated, and in addition to that they tend to produce interference. Also, some of the measuring cells are adversely influenced by radial forces, for example those which are caused by rotating eccentric loads.